Re: Something new for the New Year (2008).
Date: Tue, 08 Jan 2008 11:20:36 -0400
> On Jan 8, 1:53 am, Bob Badour <bbad..._at_pei.sympatico.ca> wrote: >
>>>On Jan 5, 7:59 pm, Rob <rmpsf..._at_gmail.com> wrote:
>>>>Now I know how Charlie Brown would feel when Lucy volunteers yet again
>>>>to hold the football for him to kick. (My apologies if you are too
>>>>young to remember Peanuts.)
>>>>Nevertheless, I believe that the path to teaching includes tolerance
>>>>and patience, so I will try to address your questions. (With some
>>>>quibbles about your logic and some questions for you about the
>>>>interpretation of foreign keys in predicates.)
>>>>>There is no difference between my and Marshall's views, because they
>>>>>are just a repetition of what Codd defined the relational model to
>>>>On January 1, JOG said:
>>>>"Tuples in databases represent facts stated in the real world (they
>>>>are not entities or objects)".
>>>>On January 3, Marshall said:
>>>>"but as far as the RM goes, it models our ideas about real-world
>>>>entities and our ideas about real-world relationships in exactly the
>>>>same way: as mathematical relations".
>>>>And on January 3, JOG said:
>>>>"A tuple represents a fact as an instantiation of a predicate."
>>>>Is see a big difference here, particularly because mathematical
>>>>relations theory says nothing about predicates.
>>>Nope no difference I'm afraid. A proposition that satisfies a
>>>predicate P(a,b,...z) is encoded as a tuple of values. All the tuples
>>>(rows) satisfying P, are collected as a set, which we call a relation
>>>(table). This is sort of database 101 really.
>>To be fair, Codd didn't establish the equivalence of expression of the
>>algebra and the calculus until his 1972 paper.
>>>>I can accept that a
>>>>tuple of a mathematical relation or a vector can be /interpreted/ as
>>>>an existence predicate, but that still leaves the question (see below)
>>>>of how a foreign key is (or is part of) a fact or proposition or
>>>>>>So as long as there is no universal consensus about how
>>>>>>relational databases and the relational model allow us to represent
>>>>>But there /is/ universal consensus from people who have read the RM
>>>>If you are speaking of Codd's 1970 paper, I've read it many times and
>>>>I /don't/ share your beliefs. In logic, "universal" has a precise
>>>>meaning. Unless you have some reason to exclude me from the set of
>>>>"people who have read the RM paper", your statement is just rhetoric.
>>>>(You could say "most" or "the majority", but you didn't.)
>>>Then you honestly need to reread the paper Rob. Or get "Introduction
>>>to Databases" by Codd, which is more thorough and more up to data. Any
>>>database theory primer should do really.
>>>>>I wonder if you are perhaps you trying to implement a different data
>>>>>model inside of the RM mechanism (as people do with EAV)? The PKFK and
>>>>>JT "structures" you describe correspond directly to propositions that
>>>>>are stated in the real world, but I am at a loss as to what facts your
>>>>>more complex A-L "structure" correspond to.
>>>>I don't know what you mean when you say that 'The PKFK and JT
>>>>"structures" you describe correspond directly to propositions that are
>>>>stated in the real world'. I'm going to take a stab at it (next), but
>>>>I would appreciate some enlightment from you on what fact or
>>>>proposition the one foreign key in a PKFK representation child tuple
>>>>stands for and what fact(s) or proposition(s) the two foreign keys in
>>>>a JT representation stand for.
>>>>IF I ASSUME THAT:
>>>>a.) a value X in the foreign key in a PKFK child tuple is the fact:
>>>>"this child tuple is related to [or included in the set of child
>>>>tuples related to] the parent tuple whose primary key value is X";
>>>>b.) a value pair (Y,Z) in the foreign keys in a JT tuple is the fact:
>>>>"the parent tuple with primary key Y is related to the child tuple
>>>>with primary key Z".
>>>>THEN FOR THE A-L REPRESENTATION:
>>>>c.) a value U in the parent foreign key of an aggregate tuple (in the
>>>>Aggregate-Link representation) is the fact: "there exists an
>>>>aggregation for which the parent tuple with primary key X has a
>>>>distinguished role", and
>>>>d.) a value pair (V,W) in the aggregate- and child foreign keys
>>>>(resp.) of a link tuple (in the Aggregate-Link representation,
>>>>associated only with the aggregate relation containing the aggregate
>>>>tuple in c) is the fact: "the child tuple with primary key W belongs
>>>>to the aggregation specified by the aggregate tuple with primary key
>>>>This is my best guess as the answer to your (implied) question 'what
>>>>facts [do] your more complex A-L "structure" correspond to?'. If I
>>>>have misinterpreted the meaning of the foreign keys that you say
>>>>'correspond directly to propositions that are stated in the real
>>>>world', I apologize. Explain what you mean and I'll try (yet) again.
>>>You're way overcomplicating Rob - you sound like an OO programmer who
>>>came to databases later? Am I right? You have to forget children and
>>>parents mate, they are OO concepts and make no sense in predicate
>>>logic where there is just inference (It was a wrench for me when I had
>>>to make that leap). Anyhow, here is an example for you:
>>>1) UK is in Europe => (country:UK, continent:Europe)
>>>2) Canada is in N. America => (country:Canada, continent:N.America)
>>>3) Bob is 30 and lives in the UK => (name:Bob, age:30, country:UK)
>>>4) Sarah is 28 and lives in Canada => (name:Bob, age:30, country:UK)
>>That's an odd way to represent that proposition.
> > Agh, the joys of cut and paste, a great tool unless one is both lazy / > and/ forgetful. > >
>>>5) Bob is Married to Sarah => (husband:Bob, bride:Sarah)
>>Since Sarah lives in Canada, the concepts of husband and bride might
>>need some adjustment. What happens when there are two husbands? Or two
> > I know nothing about Canada apart from there are a lot of moose, who > all play ice hockey and say 'aboot'. I'm visiting in April so I will > find out more then ;)
Only Yanks think we say aboot. Brits speak even funnier than we do.
>>>According to your definitions (1) and (2) are normal propositions (3)
>>>and (4) are PKFK and (5) is a JT. Your AL structure corresponds to no
>>>statement of fact that I can think of. Regards, J.
>>And according to RM, of course, they are all just propositions.
Received on Tue Jan 08 2008 - 16:20:36 CET